PROJECT SUMMARY Targeted therapeutics such as antibody-drug conjugates have made a significant impact in cancer therapies, reaching over $1B in global sales in 2016. Still, poor tumor penetration limits broad application of antibody- drug conjugates in cancer therapy. Peptide-drug conjugates (PDCs) are an attractive and promising alternative to antibody-drug conjugates for treating solid tumors with their smaller size and more cost- effective production. In vivo panning of bacteriophage peptide libraries has yielded several peptide ligands that are now in clinical trials. However, due to the relatively large size of bacteriophage (length of 880 nm for M13), the peptide ligands discovered from this approach primarily target receptors expressed on endothelial cells. The overall goal of this project is to develop a new platform for in vivo peptide screening based on synthetic nucleocapsids. Synthetic nucleocapids are engineered proteins that self-assemble into icosahedral cage-like structures containing their own genome with diameters ~ 25 nm. We recently developed this technology and demonstrated via in vivo evolution a structure that has serum stability and extended circulation half-life in mice. We therefore hypothesize that synthetic nucleocapsids are an ideal platform for peptide display and in vivo selection. The specific aims of this proposal are to optimize peptide display on the nucleocapsids, to build and characterize peptide libraries using nucleocapsids, and to determine the biodistribution, circulation and tumor penetration of nucleocapsids and nucleocapsid libraries. Successful completion of this project will lead to a new peptide discovery tool for the community that is easily transferrable and adaptable with particular advantages for in vivo use.